Engineering seed dormancy by the modification of zeaxanthin epoxidase geneexpression

Abscisic acid (ABA) is a plant hormone synthesized during seed development that is involved in the induction of seed dormancy. Delayed germination due to seed dormancy allows long-term seed survival in soil but is generally u ndesirable in crop species. Freshly harvested seeds of wild-type Nicotiana plumbaginifolia plants exhibit a clear primary dormancy that results in del ayed germination, the degree of primary dormancy being influenced by enviro nmental culture conditions of the mother plant. In contrast, seeds, obtaine d either from ABA-deficient mutant aba2-s1 plants directly or aba2-s1 plant s grafted onto wild-type plant stocks, exhibited rapid germination under al l conditions irrespective of the mother plant culture conditions. The ABA b iosynthesis gene ABA2 of N. plumbaginifolia, encoding zeaxanthin epoxidase, was placed under the control of the constitutive 35S promoter. Transgenic plants overexpressing ABA2 mRNA exhibited delayed germination and increased ABA levels in mature seeds. Expression of an antisense ABA2 mRNA, however, resulted in rapid seed germination and in a reduction of ABA abundance in transgenic seeds. It appears possible, therefore, that seed dormancy can be controlled in this Nicotiana model species by the manipulation of ABA leve ls.